The present invention concerns a compact, high-precision positioning mechanism, and, in particular, a compact, high-precision positioning mechanism that is suitable for positioning objects in scanning probe microscopes, moving-medium type memory devices, micro-manipulators and the like.
A scanning probe microscope determines the surface topology of a sample by measuring a physical parameter that exists between a probe tip and the surface of the sample. The parameter may be, for example, the capacitance or contact voltage between the probe tip and the surface, or the magnitude of a tunnelling current that flows between the probe tip and the surface. Such a scanning probe microscope is capable of measuring the physical parameter peculiar to an extremely small region of the surface of the sample. This region may have dimensions of the order of a few tens of nanometers. A moving medium-type memory device based on a scanning probe microscope can record a single bit in a region having a diameter of approximately 30 nm. A micro-manipulator based on a scanning probe microscope can work on a scale of several tens of nanometers.
In the type of scanning probe microscope in which the object whose surface topology is to be measured is caused to move relative to the probe tip, the object is typically mounted on a stage and the stage is moved. Moving the stage moves the object the same distance. Scanning probe microscopes require that the position of the object whose surface topology is to be measured, or the position of the stage, deviate from its target position by a distance that is smaller than the working scale of the microscope. A moving medium-type memory device requires that the position of the stage on which the memory medium is mounted deviate from its target position by a distance that is substantially smaller than the linear dimensions of the region of the memory medium in which one bit is recorded. Conventional positioning devices that perform this type of positioning may use piezoelectric elements as actuators. See, for example, U.S. Pat. No. 5,297,132 of Tagawa et al.
Conventional positioning devices that perform the type of positioning just described, such as the positioning device disclosed in the above-mentioned U.S. Pat. No. 5,297,132, may use piezoelectric elements as actuators. FIG. 1 shows a conventional positioning mechanism that uses stack-type piezoelectric elements as its actuators. The positioning mechanism shown in FIG. 1 is composed of the square stage 1, the inner frame 3 surrounding the stage 1, and the outer frame 5 surrounding the inner frame 3. The x-direction stack-type piezoelectric elements 7 and 9 are interposed between the stage and the inner frame, and the y-direction stack-type piezoelectric elements 11 and 13 are interposed between the inner frame and the outer frame. The inner suspension springs 15 disposed between the stage and the inner frame support the stage in the inner frame, The outer suspension springs 19 disposed between the inner frame and the outer frame support the inner frame in the outer frame. The x-direction piezoelectric elements 7 and 9 are located at the centers of two sides of the inner frame that are opposite one another in the x-direction. The y-direction piezoelectric elements 11 and 13 are located at the centers of two sides of the inner frame that are opposite one another in the y-direction. The sides of the inner frame on which the y-direction piezoelectric elements are located are orthogonal to the sides of the inner frame on which the x-direction piezoelectric elements are located.
The inner suspension springs 15 disposed between the stage 1 and the inner frame 3 are located on different opposed sides from those on which the x-direction piezoelectric elements 7 and 9 located. Two suspension springs are located on each of the opposed sides. The outer suspension springs 19 disposed between the inner frame and the outer frame 5 are located on different opposed sides from the opposed sides on which the y-direction stack-type piezoelectric elements 11 and 13 are located. Again, two suspension springs are located on each of the opposed sides.
The x-direction piezoelectric elements 7 and 9 cause the stage 1 to move in the x-direction shown in FIG. 1, and the y-direction piezoelectric elements 11 and 13 cause the stage and the inner frame 3 to move together in the y-direction shown in the same figure. Generally, stack-type piezoelectric elements require pre-loading to enable them to operate with adequate precision. Such pre-loading can be applied using springs (not shown), for example, located so that they resist the expansion and contraction forces generated by the piezoelectric elements. In the example shown in FIG. 1, pre-loading is generated simultaneously with the movement of the stage 1 or the inner frame 3 by pressing the x-direction stack-type piezoelectric elements 7 and 9 towards the stage 1 with a constant pressure, or by pressing the y-direction stack-type piezoelectric elements 11 and 13 towards the inner frame 3 with a constant pressure. When the x-direction stack-type piezoelectric elements 7 and 9 cause the stage 1 to move in the +x-direction, the piezoelectric element 7 is driven to cause it to expand, and the piezoelectric element 9 is driven to cause it to contract.
Movement of the stage 1 to any desired position in the XY plane in the conventional positioning mechanism described above requires that two stack-type piezoelectric elements be provided for each dimension of movement. In the example shown in FIG. 1, the stage is moved in two dimensions, so requires a total of four stack-type piezoelectric elements 7 through 13. A stack-type piezoelectric element can typically provide a maximum expansion or contraction of approximately 0.1% of the overall length of the element in the direction of movement. For example, a piezoelectric element with an overall length of 10 mm will provide a maximum expansion or contraction of approximately 10 xcexcm. Accordingly, to move the stage 1 by approximately xc2x150 xcexcm requires a stack-type piezoelectric element with a length of approximately 50 mm. Moreover, the length of the stack-type piezoelectric elements is aligned parallel to the direction of movement, so that the lengths of the stack-type piezoelectric elements add to the dimensions of the stage and the inner frame. The result of this is that the overall dimensions of a conventional positioning mechanism using stack-type piezoelectric elements for actuators exceed 100 mm. Consequently, the conventional positioning device just described is unsuitable for use in moving medium-type memory devices, where small size is especially required.
The suspension springs 19 should have a high compliance in their directions of movement and their compliance in the z-direction should be sufficiently low to enable them to support the moving structure composed of the inner frame 3, the stage 1 and the two stack-type piezoelectric elements 7 and 9 with little deflection in the z-direction. This requires that the suspension springs have a large thickness in the z-direction, and a very small width in the direction of movement. Suspension springs with such large aspect ratio are very difficult to fabricate. Furthermore, the mass of a moving structure having such large dimensions reduces the self-resonance frequency of the positioning mechanism. This makes it impossible to change the position of the stage 1 rapidly.
The use of lever and hinge structures that magnify the displacement provided by stack-type piezoelectric elements has been proposed to increase the range of movement provided by such actuators. However, such structures are complex and have low rigidity so that positioning mechanisms that employ them have a low self-resonance frequency. Such positioning mechanisms are therefore unsuitable for high-speed operation. Moreover, the levers used in such mechanisms generate unnecessary angular moments in directions different from the direction of movement. This reduces the precision of the positioning that can be obtained.
What is needed is a positioning mechanism that provides a range of movement of at least several tens of microns for use in scanning probe microscopes, moving medium-type memory devices, micro-manipulators and the like. This range of movement should be provided without increasing the overall size of the positioning mechanism. Moreover, it should be possible to form the positioning mechanism, including the movable element as an integral unit in a single substrate to reduce the cost mass-production.
The invention provides a positioning mechanism for positioning in a direction of movement. The positioning mechanism comprises a first member and a second member movably mounted relative to one another, and additionally comprises an elongate bending element disposed between the first member and the second member, perpendicular to the direction of movement. The elongate bending element includes two end portions attached to the second member. A portion of the elongate bending element other than the end portions is coupled to the first member. The elongate bending element bends lengthways in response to an applied driving signal to move the first member relative to the second member in the direction of movement.
The invention provides a positioning mechanism that has a range of movement of several tens of microns. The positioning mechanism may be used in such applications as a scanning-probe microscope, a moving medium-type memory device and a micro-manipulator. This range of movement is obtained without increasing the overall size of the apparatus in which the positioning mechanism is used. The compact size and large range of movement of the positioning mechanism are the result of using elongate bending elements disposed perpendicular to the direction of movement as actuators. These advantages are particularly the result of using bimorph bending elements as actuators.
The moving member of the positioning member is preferably supported by suspension springs. Electrically-conductive tracks can be located on the surface of the suspension springs to enable signals originating in the movable member to be connected to a circuit located outside the movable member.
The positioning device can be mass produced at low cost by forming the frame(s), suspension springs and movable member as an integral unit in a single substrate. The preferred material of the substrate is single-crystal silicon or a similar material.
A moving medium-type memory device that has the positioning mechanism built into its memory substrate or its probe substrate can have a high bit density. The bit density can be high because the effects of thermal expansion on the bit positions in the memory medium can be minimized by making the probe substrate and the memory substrate from the same material, such as single-crystal silicon. Because of the small size and light weight of the moving parts of the positioning mechanism, and because the elongate bending elements used in the positioning mechanism generate negligible forces in directions perpendicular to the direction of movement, the gap between the probe substrate and the memory substrate can be reduced to less than about two microns. Such a small gap further reduces the effects of thermal expansion and enables the bit density to be further increased.
In a moving medium-type memory device that has a positioning mechanism built into one of its substrates, and in which the positioning mechanism includes one elongate bending element for each of two orthogonal directions of movement, the probe substrate can be offset relative to the elongate bending elements mounted in the stage substrate. This reduces the size of the memory device.